Ontogenetic niche shifts have played an important role in the life history and ecological functions of fish. The clearhead icefish (Protosalanx chinensis, Basilewsky, 1855) is a small, pelagic, and commercially important fish that mainly feeds on zooplankton and will transition to feeding on fish when prey fish are available, though its life span is only about one year. In fact, we know little about the food selectivity and diet shifting of P. chinensis before its transition to feeding on fish. To reveal the food selectivity of P. chinensis before its transition to feeding on fish, the gut contents and environmental zooplankton community were investigated simultaneously in Lake Dalong of Northeastern China from April to June 2021. The results showed that P. chinensis experienced a diet shift from copepoda to cladocera during spring in Lake Dalong. From April to early June, both the size of cladocera and copepoda in guts increased as the size of P. chinensis increased. However, the favorite category changed to the smaller cladocera in late June, when the density of the smaller cladocera was rather high relatively. Considering June was the critical period for P. chinensis to prepare for transitioning to feeding on fish, the food resources availability must be seriously considered for sustainable aquaculture of P. chinensis. It was hypothesized that there was a trade-off of feeding selectivity between the size and density of the prey.
In order to understand the relative strength of top-down and bottom-up control in lakes of grazing alien fish, Protosalanx chinensis, investigations were designed in Lake Longhu (Lake L, P. chinensis introduced in 2013) and Lake Qijia (Lake Q, P. chinensis free). Plankton samples were collected bimonthly through the whole life cycle of P. chinensis (from February to December in 2018). A total of 133 phytoplankton and 68 zooplankton species were recorded in the two lakes. The total density and biomass of phytoplankton and zooplankton were 991.89 × 104 ind/L and 9.2418 mg/L as well as 5212 ind/L and 20.2646 mg/L, respectively. This study revealed that P. chinensis grazing did not deplete the zooplankton resources in the lake where it was stocked. Biodiversity in Lake L was not significantly different from that in Lake Q based on both phytoplankton and zooplankton. Overall, the over grazing of P. chinensis was not found in Lake L. Nevertheless, compared to Lake Q, the correlation between phytoplankton and zooplankton was weakened in Lake L, which meant there were still some effects of stocking P. chinensis on the ecological equilibrium of the plankton community, although no dramatic influences were found in Lake L yet. We also found that P. chinensis and Cladocera were significantly correlated, which should account for the top-down influences. Long-term successive investigations are suggested for sustainable resource utilization and maintaining biological balance.
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